Here is an example of above ground storage:
- Landscape Area ( lawn or garden beds)
- Impervious Area (car parking/driveway areas, paved storage yards)
- Flat roof
- Surface Tanks (Retaining wall that store water, fences)
Above Ground Storage
Above Ground Storage as Landscape area
Above Ground Storage as retaining wall / fences
Above Ground Storage (Flat Roof)
The main advantages of above ground storage are
- these can generally be easily incorporated into the site by slightly regarding or modification to the design of surface
- relatively inexpensive compared to below ground storage.
Maximum Storage Depth
Maximum storage depths in above-ground storage should not exceed the values provided in Table 19.2 .
Table 19.2 Recommended Maximum Storage Depths For Different Classes Of Above-Ground Storage
The minimum design requirements for storage systems provided in landscaped areas are:
- Maximum ponding depths should not exceed the limits recommended in Table 19.2
- 20% should be increased from the calculated storage to compensate the losses due to vegetation growth over time and the inaccuracies in construction
- The minimum ground surface slope should be 2% to promote free surface drainage and to avoid water stagnant after the pools is dry
- Maximum side slope should be 1(V) : 4(H). But if it is unavoidable, retaining walls should be provided and the safety aspects should consider (fencing the pond)
- Subsoil drainage should be provided around the outlet in order to remove excessive water and to prevent the ground becoming saturated during prolonged wet weather
- If the storage is located in an area where frequent ponding which create maintenance problems or inconvenience to property owners, it is recommended to design the storage as shown in Table 19.1. If this is not practicable, the first 10-20% of the total storage should be design in an area where it able to tolerate frequent inundation such as paved outdoor entertainment area, a permanents water feature or as rock garden.
- Landscaping should be design properly so that loose materials (mulch and bark) will not wash into the storage outlet and cause the outlet blocked.
- retaining walls shall be designed to be structurally adequate for hydrostatic loads which caused by a full storage
Table 19.1 Relative proportions for composite storage systems
Several example of impervious area which can be used for stormwater detention are:
- Car parks
- Pedestrian area
- Paved storage yards
The minimum design requirements for storage systems provided in impervious area shall be as follows:
- To avoid damage to vehicles, depths of ponding on driveway and car parks shall not exceed the limits recommended in Table 19.1.
- Transverse paving slopes within storage areas shall not be less than 0.7%
- If the storage is located in an area where frequent ponding which create maintenance problems or inconvenience to property owners, it is recommended to design the storage as shown in Table 19.1. If this is not practicable, the first 10-20% of the total storage should be design in a non sensitive area on the site.
Rooftop storage may be provided on buildings with flat roofs
Stormwater can be detained up to the maximum depth recommended in Table 19.2 by installing flow restrictors on roof drains.
A typical flow restrictor on a roof drain is shown in Figure 19.6. The degree of flow control is determined by the size and number of holes in the ring. When the water depth reaches the top of the ring, it then spills freely into the roof drain with virtually no further restriction. Water ponding depth is thereby controlled to the permissible depth while providing a controlled release rate for a measured storage volume.
Figure 19.6 Typical Roof Storage Flow Restrictor
The most common problem with rooftop detention are:
- Lack of proper inspection and maintenance
- The flow ring is clog with debris (leaves, plastic bags, newspaper) and cause the water to pond for prolonged periods
Surface tanks are normally provided on residential lots for rainwater harvesting.
These tanks collect rainwater from the rooftops of the buildings and store it for later domestic use.
Figure 19.7 Typical Multi-Purpose Surface Tank
Since surface tanks only provide detention volume for rooftops of the buildings, other forms of the detention storage (such as landscaped storage or pipe packages) must also be provided if flows from the whole site are to be reduced.
Editor :- Dilah – 13 Feb 14